Week 3 Session 7

Compare the different motors used in EV and list down the advantages and disadvantages of each.

1. DC Series Motor
2. Brushless DC Motor
3. Permanent Magnet Synchronous Motor (PMSM)
4. Three Phase AC Induction Motors
5. Switched Reluctance Motors (SRM)

DC Series Motor: High starting torque capability of the DC Series motor makes it a suitable option for traction application. It was the most widely used motor for traction application in the early 1900s. The advantages of this motor are easy speed control and it can also withstand a sudden increase in load. All these characteristics make it an ideal traction motor. The main drawback of DC series motor is high maintenance due to brushes and commutators. These motors are used in Indian railways. This motor comes under the category of DC brushed motors.

Advantage

• It gives the maximum torque at low speed.

Disadvantages

• Bulky structure
• Low efficiency
• Heat generation at brushes

Brushless DC Motor: It is similar to DC motors with Permanent Magnets. It is called brushless because it does not have the commutator and brush arrangement. The commutation is done electronically in this motor because of this BLDC motors are maintenance free. BLDC motors have traction characteristics like high starting torque, high efficiency around 95-98%, etc. BLDC motors are suitable for high power density design approach. The BLDC motors are the most preferred motors for the electric vehicle application due to its traction characteristics. You can learn more about BLDC motors by comparing it with normal brushed motor.

Advantages

• No rooter copper less
• More efficiency than the induction motors
• It is lighter
• Smaller
• Better heat dissipation
• More reliable

Disadvantages

• Short constant power range
• When speed increases torque decreases
• Expensive

Permanent Magnet Synchronous Motor (PMSM): This motor is also similar to BLDC motor which has permanent magnets on the rotor. Similar to BLDC motors these motors also have traction characteristics like high power density and high efficiency. The difference is that PMSM has sinusoidal back EMF whereas BLDC has trapezoidal back EMF. Permanent Magnet Synchronous motors are available for higher power ratings. PMSM is the best choice for high performance applications like cars, buses. Despite the high cost, PMSM is providing stiff competition to induction motors due to increased efficiency than the latter. PMSM is also costlier than BLDC motors. Most of the automotive manufacturers use PMSM motors for their hybrid and electric vehicles. For example, Toyota Prius, Chevrolet Bolt EV, Ford Focus Electric, zero motorcycles S/SR, Nissan Leaf, Hinda Accord, BMW i3, etc use PMSM motor for propulsion.

Advantages

• can be used without using gear systems in a different speed range
• More efficient
• Compact
• Best suitable for in-wheel application
• Gives high torque even at least speeds

Disadvantages

• During the in-wheel operation at high speed it losses the high amount of iron

Three Phase AC Induction Motors: The induction motors do not have a high starting toque like DC series motors under fixed voltage and fixed frequency operation. But this characteristic can be altered by using various control techniques like FOC or v/f methods. By using these control methods, the maximum torque is made available at the starting of the motor which is suitable for traction application. Squirrel cage induction motors have a long life due to less maintenance. Induction motors can be designed up to an efficiency of 92-95%. The drawback of an induction motor is that it requires complex inverter circuit and control of the motor is difficult.

Advantages

• It is the most mature commutator less motor drive system
• By employing the field operation control it can be operated like separately excited DC motor

Disadvantages

• Complex speed control

Switched Reluctance Motors (SRM): Switched Reluctance Motors is a category of variable reluctance motor with double saliency. Switched Reluctance motors are simple in construction and robust. The rotor of the SRM is a piece of laminated steel with no windings or permanent magnets on it. This makes the inertia of the rotor less which helps in high acceleration. The robust nature of SRM makes it suitable for the high speed application. SRM also offers high power density which are some required characteristics of Electric Vehicles. Since the heat generated is mostly confined to the stator, it is easier to cool the motor. The biggest drawback of the SRM is the complexity in control and increase in the switching circuit. It also has some noise issues. Once SRM enters the commercial market, it can replace the PMSM and Induction motors in the future.

Advantages

• It is simple
• Robust construction
• High speed
• Low cost
• Constant long power range
• Fault-tolerant
• Higher power density

Disadvantages

• Makes so much of noise
• Low efficiency
• Larger
• Heavy
• Complex and design control
• Problem creating in manufacturing and controllability
• Low power