Motor torque and speed characteristics in EV Technology - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Torque-Speed Relationship

Which statement best describes the general relationship between torque and speed in an electric motor?

A Torque remains constant regardless of speed.

B Torque decreases as speed increases, typically inversely proportional.

C Torque increases as speed increases linearly.

D Torque and speed are unrelated in electric motors.

💡 Hint

Think about how motors behave when starting versus running at high speed.

In most electric motors, torque is highest at low speeds and decreases as speed increases. This inverse relationship is common because the motor produces maximum torque at stall and less torque at higher speeds.

📋 Factual

intermediate

Torque at stall Condition

What is the torque called when an electric motor is at zero speed but producing maximum torque?

A Breakdown torque

B Rated torque

C Stall torque

D Running torque

💡 Hint

It is the torque when the motor shaft is not moving but the motor tries to start turning.

Stall torque is the maximum torque an electric motor can produce when the rotor is not turning (speed is zero). It represents the motor's ability to start moving a load.

🔍 Analysis

advanced

Interpreting Torque-Speed Curve

Given a motor torque-speed curve where torque drops sharply after a certain speed, what does this indicate about the motor's operating region?

A The motor is in the constant torque region before the drop.

B The motor is overheating and losing torque.

C The motor is stalled.

D The motor is in the constant power region after the drop.

💡 Hint

Consider how torque and power relate at high speeds.

When torque decreases sharply after a certain speed, the motor typically enters the constant power region where power output remains steady but torque reduces as speed increases.

❓ Comparison

advanced

Comparing Torque Characteristics of Motor Types

Which motor type generally provides high starting torque and a nearly linear decrease in torque with speed?

A Series-wound DC motor

B Brushless DC motor

C Synchronous motor

D Induction motor

💡 Hint

Think about motors used in applications requiring strong start-up force like electric vehicles.

Series-wound DC motors provide high starting torque and their torque decreases almost linearly as speed increases, making them suitable for heavy load starts.

❓ Reasoning

expert

Effect of Load on Motor Speed and Torque

If an electric motor is connected to a heavier load, what is the expected effect on its speed and torque?

A Speed decreases and torque increases.

B Both speed and torque decrease.

C Both speed and torque increase.

D Speed increases and torque decreases.

💡 Hint

Consider how motors respond to heavier loads in real machines.

When a heavier load is applied, the motor slows down (speed decreases) and produces more torque to overcome the load.

Practice

1. What does torque in an electric motor represent?

easy

A. The twisting force that makes the motor shaft rotate

B. The speed at which the motor shaft spins

C. The electrical power consumed by the motor

D. The temperature of the motor during operation

Solution

1. Step 1: Understand the definition of torque

   Torque is the force that causes rotation around an axis, in this case, the motor shaft.

2. Step 2: Differentiate torque from speed and power

   Speed is how fast the shaft spins, power is the product of torque and speed, so torque specifically means the twisting force.

3. Final Answer:

   The twisting force that makes the motor shaft rotate -> Option A

4. Quick Check:

   Torque = twisting force [OK]

Hint: Torque means twisting force, not speed or power [OK]

Common Mistakes:

• Confusing torque with speed
• Thinking torque is power output
• Mixing torque with electrical current

2. Which of the following correctly describes the relationship between torque and speed in a typical electric motor?

easy

A. Torque and speed are always equal

B. Torque increases as speed increases

C. Torque decreases as speed increases

D. Torque and speed are unrelated

Solution

1. Step 1: Recall the typical torque-speed curve for electric motors

   In many motors, torque is highest at low speed and decreases as speed increases.

2. Step 2: Understand the inverse relationship

   As the motor spins faster, the torque it can provide usually drops, showing an inverse relationship.

3. Final Answer:

   Torque decreases as speed increases -> Option C

4. Quick Check:

   Torque and speed have inverse relation [OK]

Hint: Torque usually drops when speed goes up [OK]

Common Mistakes:

• Assuming torque grows with speed
• Thinking torque equals speed
• Ignoring the motor's torque-speed curve

3. Consider a motor running at 1000 RPM producing 50 Nm torque. If the speed doubles to 2000 RPM and torque halves, what happens to the motor's power output?

medium

A. Power doubles

B. Power stays the same

C. Power halves

D. Power quadruples

Solution

1. Step 1: Calculate initial power

   Power = Torque x Speed. Initial power = 50 Nm x 1000 RPM = 50,000 (units simplified).

2. Step 2: Calculate new power after changes

   New torque = 50 / 2 = 25 Nm, new speed = 1000 x 2 = 2000 RPM. New power = 25 Nm x 2000 RPM = 50,000.

3. Final Answer:

   Power stays the same -> Option B

4. Quick Check:

   Power = Torque x Speed = constant [OK]

Hint: Power = torque x speed; doubling speed and halving torque keeps power same [OK]

Common Mistakes:

• Multiplying changes incorrectly
• Forgetting to halve torque
• Assuming power changes linearly with speed only

4. A motor's torque-speed graph shows torque dropping linearly from 100 Nm at 0 RPM to 0 Nm at 3000 RPM. If the motor is running at 1500 RPM, what is the torque? Identify the error in this calculation: Torque = 100 - (1500 / 3000) x 100 = 50 Nm.

medium

A. The calculation is correct

B. The torque should be 75 Nm, not 50 Nm

C. The torque should be 25 Nm, not 50 Nm

D. The torque cannot be calculated from this graph

Solution

1. Step 1: Understand the linear torque-speed relation

   Torque decreases from 100 Nm at 0 RPM to 0 Nm at 3000 RPM, so torque at any speed is 100 x (1 - speed/3000).

2. Step 2: Calculate torque at 1500 RPM

   Torque = 100 x (1 - 1500/3000) = 100 x 0.5 = 50 Nm, matching the given calculation.

3. Final Answer:

   The calculation is correct -> Option A

4. Quick Check:

   Torque at half speed = half torque [OK]

Hint: Torque halves at half max speed in linear drop [OK]

Common Mistakes:

• Misapplying the formula
• Confusing RPM ratio direction
• Assuming torque is constant

5. An electric vehicle motor has a torque-speed characteristic where torque is constant at 200 Nm up to 2000 RPM, then decreases linearly to 0 Nm at 6000 RPM. What is the torque at 4000 RPM?

hard

A. 50 Nm

B. 150 Nm

C. 200 Nm

D. 100 Nm

Solution

1. Step 1: Identify torque behavior zones

   Torque is constant 200 Nm up to 2000 RPM, then decreases linearly to 0 Nm at 6000 RPM.

2. Step 2: Calculate torque at 4000 RPM

   From 2000 to 6000 RPM (a 4000 RPM range), torque drops from 200 Nm to 0. At 4000 RPM, which is 2000 RPM into this range, torque = 200 x (1 - 2000/4000) = 200 x 0.5 = 100 Nm.

3. Final Answer:

   100 Nm -> Option D

4. Quick Check:

   Halfway in drop range means half torque [OK]

Hint: Calculate linear drop after constant torque zone [OK]

Common Mistakes:

• Using full RPM instead of drop range
• Forgetting constant torque zone
• Mixing RPM values in formula